Fiber optic switch and discretely variable delay line

ABSTRACT

A single mode optical fiber switch having a base (70) and a laterally slidable top (91). The base is composed of a quartz block (70) with a slight radius of curvature along its length supporting a silicon substrate (66) having a plurality of parallel v-grooves (68). The grooves are fitted with single mode optical fiber segments (64) and lapped to create a flat coupling surface (82) extending laterally across the base. The top (91) is composed of a quartz block with a single v-groove (93) holding a segment of single-mode fiber (95). This fiber (95) is lapped to create a flat coupling surface matable with the coupling surface on the base. The base (70) and top (91) are placed together such that the top fiber (95) may be selectively slid into and out of coupling alignment with any of the fibers (64) in the base (70) to form a discretely variable delay line. A single length of fiber (121) is wrapped around the base ( 70) in helical fashion with each loop being secured in one of the v-grooves (68). By coupling the top fiber (95) with different loops of the base fibers (121), different amounts of delay can be obtained. The delay line can be used to provide variable frequency response for notch and transversal filters.

BACKGROUND OF THE INVENTION

This invention relates to the processing of optical signals transmitted through optical fibers. More particularly the invention includes multiple optical fibers in a closely spaced side-by-side array on a substrate with an optical tap on each of the fibers. A second substrate with a tapped optical fiber is positioned in a face-to-face relationship with the first substrate. The second substrate is laterally adjusted so that the optical tap in the fiber on the second substrate may be selectively aligned with any one of the optical taps on the first substrate.

If the optical fibers on the first substrate are independent from one another the structure yields the optical equivalent of a single pole, multiple throw electrical switch which permits selective tapping of information into the fiber on the second substrate from any one of the fibers on the first substrate. Alternatively, the structure will selectively tap information from the fiber mounted on the second substrate into any one of the fibers mounted on the first substrate. If, in an alternative configuration, the first substrate mounts successive portions along the length of a single fiber, the apparatus forms a discretely variable fiber optic delay line.

Fiber optic switches, particularly for multi-mode fibers, are known in the art. Typically such switches are constructed by mounting multiple fibers on a first substrate such that cleaved or polished ends of the fibers are coplanar with one surface of the substrate. A second substrate mounts a fiber in a similar manner. The surfaces of the two substrates may be accurately transposed in order to align the butt end of the fiber in the second substrate with a selected one of the butt ends of the fibers in the first substrate, thus generating a switch. If such techniques were attempted with single mode optical fibers the degree of alignment required for low loss coupling would be extremely difficult to achieve. Thus, for example, if the core diameter of a single mode optical fiber is four microns, a misalignment of the two substrates by four microns would substantially eliminate coupling between the respective fibers. For this reason, a practical, less position-sensitive switch mechanism is needed, particularly for single mode optical fibers.

With regard to the use of this device as a delay line, it will be recognized that the advantages of fiber optic delay lines are well-known in the art. Thus, for example, tranversal filters capable of selectively filtering modulated light signals has been taught. Furthermore, the construction of transversal filters by helically wrapping a single fiber optic element around a series of v-grooves in a silicon chip, with taps at each groove, is known in the art. However, because no adjustment of the delay line lengths has been possible in prior art delay lines or transversal filters, the frequency vs. attentuation characteristics of prior transversal filters was, of necessity, determined at the time of construction of the filter. There is therefore a need for an adjustable fiber optic delay line so that, for example, the frequency response of a transversal filter utilizing the delay line may be adjusted.

SUMMARY OF THE INVENTION

The present invention provides a single mode fiber optic switch which is capable of coupling nearly one-hundred percent of the light energy from an input fiber to any one of multiple output fibers, or, alternatively, nearly one-hundred percent of the input light energy from any one of multiple input fibers to an output fiber. This switch structure is useful for the construction of a discretely variable delay line wherein the multiple input fibers are successive length portions of a single optical fiber. In the latter case, for example, the discretely variable delay line may be used to provide a transversal filter with a variable frequency response.

The structure of this invention comprises a plurality of single mode optical fibers mounted in a side-by-side array in parallel v-grooves in a plate or chip. In the case of a fiber optic switch these optical fibers are independent of one another. In the case of a discretely variable delay line, the fibers are successive lengths of a single optical fiber which is wrapped around the plate or chip so that successive portions are in adjacent v-grooves. A portion of the cladding on each optical fiber mounted on the plate or chip is removed along a lateral line normal to the length of the fiber in the v-grooves, thereby simultaneously creating a tap in each fiber. Light is selectively detected from one of the taps by superimposing on the plate or chip, a second v-grooved plate or chip which supports a single optical fiber, the cladding of which has been similarly removed. When the fibers are superimposed, evanescent field coupling occurs between the fiber on the second plate or chip and a selected one of the fibers on the first plate or chip. This selection depends upon the relative position of the two plates. By adjusting the lateral position of the second plate with reference to that of the first, light can be selectively coupled between the tapped area of the fiber in the second substrate and any one of the tapped fibers in the first substrate.

The optical fibers on the first plate are preferably mounted so that the distances between successive fibers is identical. In addition, the level at which the plural fibers are mounted on the plate, at the location where they are to be tapped, is identical, since the taps are made by lapping a portion of the cladding from each of the fibers simultaneously. To the extent that the level of the fibers in the plate is identical, the taps can be identically formed.

In the preferred embodiment, the plate is a silicon chip having etched parallel grooves. It is possible to etch silicon chips by using photolithographic methods and to thereby obtain a high degree of accuracy.

Advantageously, the second plate is formed as a quartz block including a v-groove but may alternatively be formed as an etched silicon substrate.

Single mode optical fibers generally have a core diameter of a few microns and a cladding diameter in a range of one-hundred microns along with an outer cover. In that portion of the fiber which is placed in the silicon v-grooves, the outer cover is removed and the cladding is secured to the v-groove to permit accurate location of the core and to thereby permit polishing of the fibers to within a few microns of the core. The coupling technique provided by this switch is described as evanescent field coupling. It is therefore important that the amount of material removed from the fiber cladding be controlled so that the spacing between the core portions of abutted fiber optic strands be within a predetermined critical zone. This causes each strand to receive a significant portion of the evanescent field energy from the other strand, so that coupling is achieved without significant energy loss. It is also well-known that the amount of coupling between the two fibers is determined by the distance between the fiber cores. Additionally the amount of coupling can be varied by moving one of the cores laterally with respect to the other, so that the cores are somewhat out of alignment. However, it is important to note that the coupling between fibers in an evanescent field coupler of the type described herein is far less sensitive to lateral misalignment than would be the case where the fibers are butt-coupled. Thus, the switch of the present invention provides adequate coupling even in the case where the optical fiber cores are misaligned by several microns, a situation which would destroy the effectiveness of the butt coupled switch described earlier. Thus the present invention provides a practical switch and furthermore provides a practical discretely variable delay line not heretofore possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

These and other advantages of the present invention are best understood through reference to the drawings, in which:

FIG. 1 is a cross-sectional view of the fiber optic coupler of the present invention, showing a pair of fiber optic strands mounted in respective arcuate grooves of respective bases;

FIGS. 2 and 3 are cross-sectional views of the coupler of FIG. 1, taken along the lines 2--2 and 3--3, respectively;

FIG. 4 is a perspective view of the lower base of the coupler of FIG. 1, separated from the other base, to show its associated fiber mounted thereon, and the oval-shaped, facing surface of the fiber;

FIG. 5 is a schematic diagram showing the evanescent fields of the pair of fibers overlapping at the interaction region;

FIG. 6 is a schematic drawing of the coupler of FIG. 1, illustrating the radius of curvature, core spacing, and interaction length, as being parameters of the coupler;

FIG. 7 is a schematic drawing of an "equivalent" coupler;

FIG. 8 is a graph of normalized coupled power as a function of interaction length for a given fiber core spacing;

FIG. 9 is a graph of normalized coupled power as a function of interaction length for another fiber core spacing;

FIG. 10 is a graph of normalized coupled power as a function of minimum fiber core spacing (spacing surfaces superimposed);

FIG. 11 is a schematic representation of the oval-shaped facing surfaces of the fibers, showing the facing surfaces laterally offset;

FIG. 12 is a graph of normalized coupled power as a function of lateral offset for a first minimum fiber core spacing;

FIG. 13 is a graph of normalized coupled power as a function of lateral offset for a second fiber core spacing;

FIG. 14 is a graph of normalized coupled power as a function of lateral offset for a third fiber core spacing;

FIG. 15 is a graph of (a) normalized coupled power as a function of lateral offset and (b) throughput loss as a function of lateral offset for one exemplary coupler showing that the throughput loss is substantially constant over substantially the entire range of normalized coupled power;

FIG. 16 is an exploded perspective view of the switch of this invention;

FIG. 17 is a sectional view taken along line 17--17 of FIG. 16;

FIG. 18 is a perspective view of a portion of the grooved chip used to properly align the optical fiber windings;

FIG. 19 is a perspective view illustrating the photolithographic technique utilized to achieve precise alignment of the v-grooves;

FIG. 20 is a perspective view of a portion of the silicon wafer ready to be etched;

FIG. 21 is a perspective view of a fixture used for adjusting the switch of this invention;

FIG. 22 is an exploded perspective view of the discretely variable delay line of this invention;

FIG. 23 is a graph showing the delays for various taps of the delay line of FIG. 22;

FIG. 24 is a schematic diagram of a fiber incorporating the delay line of FIG. 22; and

FIGS. 25(a)-(c) are graphs showing the frequency response of the filter of FIG. 24.

In order to thoroughly understand the present invention, it is important to first understand the construction and operation of single mode fiber optic evanescent field couplers.

Construction of Evanescent Field Coupler

As illustrated in FIGS. 1 to 4, an evanescent coupler 10 includes two strands 12a and 12b of a single mode fiber optic material mounted in longitudinal arcuate grooves 13a and 13b, respectively, formed in optically flat, confronting surfaces 14a and 14b, respectively, of rectangular bases or blocks 16a and 16b, respectively. The block 16a with the strand 12a mounted in the groove 13 will be referred to as the coupler half 10a, and the block 16b with the strand 12b mounted in the groove 13b will be referred to as the coupler half 10b.

Each of the strands 12a and 12b comprises a commercially available fiber of quartz glass which is doped to have a central core and an outer cladding. It will be seen below that the present invention is particularly advantageous for use with single mode fibers, which typically have a core diameter on the order of 10 microns or less and a cladding diameter on the order of 125 microns, although the invention may also be employed with other types of fibers, such as multi-mode fibers. In the embodiment disclosed, single mode fibers are utilized, however, for clarity of illustration, the diameter of the strands 12 and their respective cores are exaggerated. Furthermore, test results described herein are for couplers utilizing single mode fibers.

The arcuate grooves 13a and 13b have a radius of curvature which is vary large compared to the diameter of the fibers 12, and have a width slightly larger than the fiber diameter to permit the fibers 12, when mounted therein, to conform to a path defined by the bottom walls of the grooves 13. The depth of the grooves 13a and 13b varies from a minimum at the center of the blocks 16a and 16b, respectively, to a maximum at the edges of the blocks 16a and 16b, respectively. This advantageously permits the fiber optic strands 12a and 12b, when mounted in the grooves 13a and 13b, respectively, to gradually converge toward the center and diverge toward the edges of the blocks 16a, 16b, thereby eliminating any sharp bends or abrupt changes in direction of the fibers 12 which may cause power loss through mode perturbation. In the embodiment shown, the grooves 13 are illustrated as being rectangular in cross-section, however, it will be understood that other suitable cross-sectional contours which will accommodate the fibers 12 may be used alternatively, such as a U-shaped cross-section or a V-shaped cross-section. Techniques for forming the grooves 13 and mounting the fibers 12 therein are discussed below.

At the centers of the blocks 16, in the embodiment shown, the depth of the grooves 13 which mount the strands 12 is less than the diameter of the strands 12, while at the edges of the blocks 16, the depth of the grooves 13 is preferably at least as great as the diameter of the strands 12. Fiber optic material was removed from each of the strands 12a and 12b to form the respective oval-shaped planar surfaces 18a, 18b, which are coplanar with the confronting surfaces 17a, 17b, respectively. These surfaces 18a, 18b will be referred to herein as the fiber "facing surfaces". It will be understood that these facing surfaces provide a coupling area or region for the fibers 12a and 12b. Thus, the amount of fiber optic material removed increases gradually from zero towards the edges of the block 16 to a maximum towards the center of the block 16. This tapered removal of the fiber optic material enables the fibers to converge and diverge gradually, which is advantageous for avoiding backward reflection and excess loss of light energy.

In the evanescent coupler shown, the coupler halves 10a and 10b are identical, and are assembled by placing the confronting surfaces 14a and 14b of the blocks 16a and 16b together, so that the facing surfaces 18a and 18b of the strands 12a and 12b are in facing relationship.

An index matching substance (not shown), such as index matching oil, is provided between the confronting surfaces 14. This substance has a refractive index approximately equal to the refractive index of the cladding, and also functions to prevent the optically flat surfaces 14 from becoming permanently locked together. The oil is introduced between the blocks 16 by capillary action.

An interaction or coupling region 32 is formed at the junction of the strands 12, in which light is transferred between the strands by evanescent field coupling. It has been found that, to insure proper evanescent field coupling, the amount of material removed from the fibers 12 must be carefully controlled so that the spacing between the core portions of the strands 12 is within a predetermined "critical zone". The evanescent fields extend into the cladding and decrease rapidly with distance outside their respective cores. Thus, sufficient material should be removed to permit each core to be positioned substantially within the evanescent field of the other. If too little material is removed, the cores will not be sufficiently close to permit the evanescent fields to cause the desired interaction of the guided modes, and thus, insufficient coupling will result. Conversely, if too much material is removed, the propagation characteristics of the fibers will be altered, resulting in loss of light energy due to mode perturbation. However, when the spacing between the cores of the strands 12 is within the critical zone, each strand receives a significant portion of the evanescent field energy from the other strand, and optimum coupling is achieved without significant energy loss. The critical zone is illustrated schematically in FIG. 5 as including that area, designated by the reference numeral 33, in which the evanescent fields, designated by reference numerals 34a and 34b, of the fibers 12a and 12 b, respectively, overlap with sufficient strength to provide coupling, i.e., each core is within the evanescent field of the other. However, as previously indicated, mode perturbation occurs within the area 33 when the cores are brought too close together. For example, it is believed that, for weakly guided modes, such as the HE₁₁ mode in single mode fibers, or high order modes in multi-mode fibers, such mode perturbation begins to occur when sufficient material is removed from the fibers 12 to expose their cores. Thus, the critical zone is defined as that area in which the evanescent fields 34 overlap with sufficient strength to cause coupling without substantial mode perturbation induced power loss.

The extent of the critical zone for a particular coupler is dependent upon a number of interrelated factors such as the parameters of the fiber itself and the geometry of the coupler. Further, for a single mode fiber having a step-index profile, the critical zone can be quite narrow. In a single mode fiber coupler of the type shown in FIGS. 1-4, for example, the required center-to-center spacing between the strands 12 at the center of the coupler is typically less than a few (e.g., 2-3) core diameters.

Typically, the strands 12a and 12b are (1) identical to each other, (2) have the same radius of curvature at the interaction region 32, and (3) have an equal amount of fiber optic material removed therefrom to form their respective facing surfaces 18a and 18b. Thus, the fibers 12 are symmetrical, through the interaction region 32, in the plane of the facing surfaces 18, so that the facing surfaces 18 are coextensive if superimposed. This insures that the two fibers 12a and 12b will have the same propagation characteristics at the interaction region 32, and thereby avoids coupling attenuation associated with dissimilar propagation characteristics.

The blocks or bases 12 may be fabricated of any suitable rigid material. In a typical coupler, the bases 12 comprise generally rectangular blocks of fused quartz glass approximately one inch long, one inch wide, and 0.4 inches thick. The fiber optic strands 12 are typically secured in the slots 13 by suitable cement 38, such as epoxy glue. One advantage of the fused quartz blocks 16 is that they have a coefficient of thermal expansion similar to that of glass fibers, and this advantage is particularly important if the blocks 16 and fibers 12 are subjected to any heat treatment during the manufacturing process. Another suitable material for the block 16 is silicon, which also has excellent thermal properties for this application.

Operation of the Coupler 10

The coupler 10 includes four ports, labeled A, B, C, and D in FIG. 1. When viewed from the perspective of FIG. 1, ports A and C, which correspond to strands 12a and 12b, respectively, are on the lefthand side of the coupler 10, while the ports B and D, which correspond to the strands 12a and 12b, respectively, are on the righthand side of the coupler 10. For the purposes of discussion, it will be assumed that input light of a suitable wavelength (e.g., 1.15 microns) is applied to port A. This light passes through the coupler and is output at port B and/or port D, depending upon the amount of power that is coupled between the strands 12. In this regard, the term "normalized coupled power" is defined as the ratio of the coupled power to the total output power. In the above example, the normalized coupled power would be equal to the ratio of the power at port D to the sum of the power output at ports B and D. This ratio is also referred to as the "coupling efficiency", and when so used is typically expressed as a percent. Thus, when the term "normalized coupled power" is used herein, it should be understood that the corresponding coupling efficiency is equal to the normalized coupled power times one-hundred. In this regard, tests have shown that an evanescent coupler 10 may have a coupling efficiency of up to one-hundred percent. However, it will also be seen that the coupler 10 may be "tuned" to adjust the coupling efficiency to any desired value between zero and the maximum.

Furthermore, an evanescent coupler 10 is highly directional, with substantially all of the power applied at one side of the coupler being delivered to the other side of the coupler. The coupler directivity is defined as the ratio of the power at port D to the power at port C, with the inut applied to port A. Tests have shown that the directionally coupled power (at port D) is greater than 60 dB above the contradirectionally coupled power (at port C). Further, the coupler directivity is symmetrical. That is, the coupler operates with the same characteristics regardless of which side of the coupler is the input side and which side is the output side. Moreover, the coupler 10 achieves these results with very low throughput losses. The throughput loss is defined as the ratio of the total output power (ports B and D) to the input power (port A), subtracted from one (i.e. 1-(P_(B) +P_(D))/P_(A)). Experimental results show that throughput losses of 0.2 dB have been obtained, although losses of 0.5 dB have been obtained, although losses of 0.5 dB are more common. Moreover, these tests indicate that the coupler 10 operates substantially independently of the polarization of the input light applied.

The coupler 10 operates on evanescent field coupling principles in which guided modes of the strands 12 interact, through their evanescent fields, to cause light to be transferred between the strands 12. As previously indicated, this transfer of light occurs at the interaction region 32. The amount of light transferred to dependent upon the proximity and orientation of the cores, as well as the effective length of the interaction region 32. The length of the region 32 is, in turn, dependent upon the radius of curvature of the fibers 12, and, to a limited extent, the core spacing, although it has been found that the effective length of the interaction region 32 is substantially independent of core spacing. In one typical coupler 10, employing an edge-to-edge core spacing of abot 1.4 microns, the radius of curvature is on the order of 25 centimeters, and the effective interaction region is approximately one millimeter long at a signal wavelength of 633 nm. With these dimensions, the light makes only one transfer between the strands 12 as it travels through the interaction region 32. However, if the length of the interaction region 32 is increased, or core spacing decreased, a phenomenon referred to herein as "overcoupling" will occur, in which the light will transfer back to the strand from which it originated. As the interaction length is further increased, and/or the core spacing further decreased, the light transfers back to the other strand. Thus, the light may make multiple transfers back and forth between the two strands 12 and it travels through the region 32, the number of such transfers being dependent on the length of the interaction region 32, and the core spacing.

The foregoing may be more fully understood through reference to FIG. 6 which shows a schematic representation of the coupler 10 of FIG. 1. The cores of fibers 12a and 12b are shown as gradually converging to a minimum spacing, labeled H, at the center of the coupler and diverging towards the edges of the coupler. The effective interaction length is labeled L and the radius of curvature of the strands 12a and 12b is labeled R. As indicated above, it has been found that, while the effective interaction length L is a function of the radius of curvature R, it is substantially independent of the minimum spacing H between the fibers 12. Although this independence is truly valid only for relatively large core spacings and short wavelengths, it provides a good approximation for most applications, and thus, it advantageously permits the coupler, illustrated in FIG. 6, to be analyzed as an "equivalent" coupler comprising two parallel wave guides, separated through their interaction length L, by a spacing H, as shown in FIG. 7.

The effects of varying either the effective interaction length L or the fiber spacing H of the "equivalent" coupler shown in FIG. 7 may be understood through reference to FIGS. 8 and 9. FIG. 8 shows a sinusoidal curve 40 which illustrates that the coupled power P_(C) varies sinusoidally as a function of the interaction length L for a given Fiber spacing H1. At this fiber spacing, it may be seen that the coupled power is approximately fifty-percent when the interaction length is equal to L1, and increases to one-hundred-percent when the interaction length increases L2. If the interaction length if further increased, "overcoupling" results, in which light is transferred back to the strand from which it originated, and the coupled power P_(C) begins to decrease towards zero. The coupled power then increases from zero to, for example, fifty percent at L3. Thus, it may be seen that the amount of coupling my be varied by changing the effective length of the interaction region.

The effect of decreasing the spacing H between the fibers is to increase the strength of the coupling, and thus, increase the amount of light transferred over a given interaction length L, as shown by comparing the sinusoidal curve 42 of FIG. 9 with the sinusoidal curve of FIG. 8. For example, if the fiber spacing were decreased from H₁ (FIG. 8) to H₂ (FIG. 9), the coupled power may be one-hundred percent at interaction length L1 in FIG. 9, as compared to fifty percent for the same interaction length L1 in FIG. 8. The curve 42 then begins to demonstrate overcoupling and the coupled power decreases to fifty percent at interaction length L2. At interaction length L3, the curve 42 indicates that the coupled power is again one-hundred percent. Thus, for a given interaction length (e.g., L1, L2, or L3), the amount of power coupled may be adjusted by changing the fiber core spacing.

The relationship between the minimum fiber spacing H and the coupled power P_(C) for a given interaction length L (i.e., radius of curvature) is illustrated in FIG. 10 by the curve 44. As shown in this figure, the normalized coupled power oscillates between 0 and 1 with increasing frequency or the core spacing H decreases. The reference points a, b, and c on the curve H were chosen somewhat arbitrarily to indicate normalized coupled power of 0.5, 1.0, and 0.25, respectively. It will be recognized that at point "a" fifty percent of the power is coupled from one to the other. At point "b" full coupling is achieved, and one-hundred percent of the optical power is transferred between the strands. Point "c", on the other hand, represents an overcoupled condition wherein the coupled power has decreased from full coupling to twenty-five percent.

Tuning the Coupler 10

The foregoing concepts are useful in understanding the "tunability" aspects of the coupler 10. As used herein, the term "tuning" is defined as translating the fibers 12 relative to each other to adjust the power coupled therebetween. Such translation of the fibers 12 is accomplished by sliding the planar facing surfaces 18 relative to each other, so that they are offset, rather than superimposed. That is, the fibers 12 are mutually displaced in the plane of the planar facing surfaces. Viewed another way, such translation occurs when rspective planes in which each of the fibers lie, are displaced relative to each other.

In one method of fiber translation, the facing surfaces 18 are offset laterally. As used herein, the term "laterally offset" means sliding the facing surfaces 18 laterally from their superimposed position to increase the spacing between the fiber cores, while maintaining a substantially parallel relationship between the fibers 12. Such lateral offset of the facing surfaces 18 is illustrated schematically in FIG. 11. The effect of such lateral offset is, of course, to change the spacing between the cores of the fibers 12.

The curve 46 of FIG. 12 graphically illustrates the effect of laterally offsetting the fiber facing surfaces 18 for a coupler having a minimum edge-to-edge core spacing H equal to "a" (FIG. 10). When the facing surfaces 18 of the fibers are superimposed (i.e., no offset), the normalized coupled power will be equal to 0.5, as required by the curve 44 of FIG. 10. However, when the facing surfaces of the fibers 12 are laterally offset in either direction to increase the spacing between the cores, the coupled power gradually decreases to zero.

Referring now to the curve 48 of FIG. 13, the effect of lateral fiber offset upon normalized coupled power for a coupler having an edge-to-edge core spacing equal to "b" (FIG. 10) is shown. When there is no offset and the facing surfaces 18 are superimposed, the normalized coupled power is 1.0, as required by the curve 44 of FIG. 10, however, as the facing surfaces 18 of the fibers 12 are laterally offset in either direction, the coupler power gradually decreases.

The curve 50 of FIG. 14 shows coupled power as a function of relative fiber offset for a core spacing equal to "c" (FIG. 10), which, as will be recalled, represents an overcoupled condition. From this curve 50, it may be seen that, when the facing surfaces 18 of the fibers 12 are superimposed, the normalized coupled power is 0.25. As the core spacing is increased by sliding the facing surfaces 18 so that they are laterally offset, the normalized coupled power initially increases to 1.0 and then decreases toward zero when the core spacing is further increased.

In all of the foregoing cases illustrated by FIGS. 12, 13 and 14, the coupled power decreases to zero at the substantially same lateral offset, assuming that the physical dimensions of the couplers corresponding to these figures are identical except for their fiber spacing at zero offset. By comparing the curves 46, 48 and 50 of the FIGS. 12, 13 and 14, respectively, it may be seen that the respective slopes of these curves tend to increase as the core spacing decreases. Thus, the sensitivity of the coupler to lateral offset increases as the core spacing decreases. For example, a coupler fabricated to exhibit overcoupling, as in FIG. 14, is significantly more sensitive to lateral offset than couplers having the characteristics shown in either FIG. 12 or 13. This feature of the present invention is quite advantageous, since, in the switch application of the present invention, low sensitivity and high stability are typically desirable. Thus, a coupler having coupling characteristics similar to those of FIGS. 12 and 13 (i.e., not overcoupled) is more appropriate for this application.

Experimental evidence indicates that the throughput losses of the coupler 10 are substantially constant, except when the lateral offset of the cores is relatively large. Tests on one exemplary coupler showed that the throughput loss was within 0.2 dB of the minimum loss for a lateral offset of up to 10 microns in either direction. This coupler utilized a single mode fiber having a core index of 1.460, a cladding index of 1.4559, and a core diameter of four microns. The radius of curvature of the fibers was 25 centimeters, the edge-to-edge core spacing was approximately 0.9 micrtons, and the wavelength of the light utilized was 632.8 nm. FIG. 15 shows, for this exemplary coupler, a graph of throughput loss, designated by the reference numeral 60, and a graph of the normalized coupled power, designated by the reference numeral 62, both as a function of lateral offset of the facing surfaces 18. The two horizontal broken lines drawn through the center of FIG. 14 provide the upper and lower boundaries of an 0.2 dB power loss band. It may be seen that the power loss curve 60 is within this band for lateral offsets up to about 12 microns in either direction. Further, it may be seen that, at a 12-micron lateral offset, the normalized coupled power is approximately 0.1. Thus, for coupled power between 0.1 and 1, the power loss is within about 0.2 dB of the minimum power lows. If the power loss band is expanded to 0.5 dB, the power loss band is within the 0.5-dB band for fiber offsets of up to 15 microns, which corresponds to a coupled power of less than 0.05 (i.e., 5 percent). Thus, this coupler exhibits substantially constant throughput losses, i.e., within a relatively narrow power loss band width, throughout substantially the entire operating range of the device. Further, it is significant that the throughput losses are very low, and relatively constant for coupler power between ten percent and one-hundred percent.

Construction of Fiber Optic Switch

The present invention is illustrated in FIGS. 16 and 17 which shows plural optical fibers 64 mounted on a chip 66 with individual v-grooves 68 to receive each optical fiber 64. Taps are formed by simultaneously polishing the fibers and the chip 66, so that each of the optical fibers 64 is tapped at a lateral line 70. Before a description of the manner of formation of the taps on the optical fibers 64, it is necessary to describe the construction of the chip 66 and the manner by which the optical fibers 64 are laid in the v-grooves 68 in the chip 66.

Referring to FIG. 18, a portion of the chip 66 with the completed v-grooves 68 is shown. It is desirable that each of these v-grooves 68 be identical in width and depth, since the fibers are to be coplanar because the taps in the fibers in these v-grooves will be constructed simultaneously. If the v-grooves are identical, the taps on the fibers will have uniform characteristics.

Because of this need for great precision in the construction of the v-grooves, the preferred material for the chip 66 is silicon, which can be etched by photolithographic techniques, rather than some other material in which grooves are machined. The type of silicon used is commercially available 100 oriented silicon, because when the surface of this type of silicon is subjected to an etching solution, the surface dissolves at an angle theta, as shown in FIG. 18. For 100 oriented silicon the angle theta is 54.74 degrees.

Therefore, in order to make the v-grooves shown in FIG. 18 in the silicon chip 66, the following photolithographic method may be used. The chip 66 is first heated so that a very thin oxide layer 72 will cover the chip 66, as shown in FIG. 19. The chip 66 is then coated with a photosensitive coating 74 known as photoresist. A mask 80 is then placed over the chip, and the top of the chip is exposed to ultraviolet light. For the mask 80 shown in FIG. 19, positive-acting photoresist is used, so the portions of the photoresist 74 under the opaque sections of the mask 80 will be left to perform the masking function.

The next step is to etch away the photoresist 74 and oxide layer 72 which were under the non-opaque portions of the mask 80 by using a buffered hydrochloric acid solution. The remaining photoresist is then stripped by use of a special solvent, and the chip 66 will appear as it does in FIG. 20, covered by a mask of the oxide layer 72. The silicon wafer 66 is then placed into an anisotropic etching solution, commonly potassium hydroxide solution. The portions of the chip no covered by the oxide layer 72 will then be dissolved by the etching solution at an angle of 54.74 degrees from the surface 76 of the chip 66. The etch will develop as a flat-bottom groove until the two angled planes intersect and a true v-groove is formed, as shown in FIG. 18. This photolithographic technique is well-known in the art, and has been used extensively in the manufacture of semiconductors.

The size of the groove 68 to be etched in the silicon chip 66 of course, depends on the optical fiber being used. There are two widely-used types of single mode optical fiber, one manufactured by ITT which has a diameter of about 80 microns (400 microns including the plastic jacket), and a second brand of fiber manufactured by Corning which is 125 microns in diameter (135 microns with the lacquer jacket). Which of these or other fibers is used depends on the wavelength of the light to be used; if visible light is to be used, the ITT fiber is acceptable since it is single mode at visible light wavelengths (longer than green); and if infrared light is to be used, the Corning fiber is acceptable since it is single mode at infrared wavelengths.

Referring again to FIG. 16, in order to construct optical taps on the fibers 64, it is desirable to have the optical fibers 64 mounted in a curved configuration so that a selected portion of the optical fibers 64 may be lapped. Therefore, the silicon chip 66 is mounted on a block of quartz 70 which has a curved upper surface 72. It has been found that a silicon chip 66 of standard thickness (0.25 millimeter) and about three-centimeter length can be bent without breaking around a twelve-inch radius. An adhesive substance such as wax is used to hold the silicon chip 66 on the quartz block 70 in the curved configuration shown in FIG. 16.

Before the optical fibers 64 can be mounted in the v-grooves 68 in the silicon chip 66, the portions of the optical fibers 64 which are to be mounted in the v-grooves 68 preferably have the protective jacket removed. The ITT fiber has a plastic jacket which may be removed by dipping it in sulfuric acid. The resiliency of the plastic jacket prevents precision in the lapping operation, so it should be removed. The Corning fiber has a lacquer jacket which may be removed by dipping it in acetone. Since the lacquer jacket is more solid than the plastic jacket, its removal is preferential rather than mandatory.

The installation of the fibers 64 in the v-grooves 68 of the silicon chip 66 is best shown in FIG. 17. An adhesive 77 is placed in the bottom of each v-groove 68. An optical fiber 64 is then placed in each v-groove 68 and placed in tension so that the optical fiber 64 bottoms out against the sides of the v-groove 68 to be permanently retained there by the adhesive 76.

The next step is to lap the upper surface of the optical fibers 64 at the lateral line 70. This lapping operation will remove a portion of the cladding 79, as best shown in FIG. 17. It is important to note that not all of the cladding material 76 around the core 78 of the optical fiber 64 is to be removed. The distance between the core 78 and the lapped surface 82 depends on the characteristics of the coupler to be formed, in accordance with the previous description. The quartz block 70 advantageously includes plural spacing elements 22 which are used to stabilize the chip 66 during polishing and to provide a bearing surface for the second half of the switch.

The second half of the switch is preferably formed of a quartz block 91 including a groove 93 as earlier described for a standard coupler. As can be seen from FIG. 16 the groove 83 has a relatively large radius of curvature, peaking at the center of the substrate 91. An optical fiber 95 is placed within the groove 93 and secured using epxoy or any other satisfactory adhesive. This fiber is then lapped at the highest point of the curve along with the surface of the quartz block 91 to yield a planar surface for the cladding, which is a few microns separated from the core of the fiber, and which is co-planar with the surface of the quartz block 91.

An index matching oil is then placed on the polished flat surface 82 of the silicon substrate 66. In this case, the index matching oil advantageously has an index of refraction slightly lower than that of the cladding 79 of the fiber elements 64 and the fiber cores 78 themselves to prevent loss of light outside of the coupling area.

If the fibers 64 on the lower substrate 66 are independent fibers, the construction shown in FIG. 16 provides an optical switch. Switching is accomplished by aligning the core of the upper fiber 95 with the desired lower fiber 64 core. When a different fiber 64 on the lower substrate 66 is to be selected as the input or output fiber, an operator merely adjusts the position of the upper quartz block 91 in the lateral direction (arrow 97) relative the axis of the fiber 95 with respect to the lower substrate 66 so that the upper fiber 95 is in alignment with the desired lower fiber 64. Adjustable coupling with respect to each of the fibers 64 can be obtained by positioning the upper substrate 91 such that the fiber 95 is slightly offset from the selected fiber 64. Also, as previously mentioned, coupling can be varied by orienting the upper fiber 95 at an angle with respect to the desired lower fiber 64. In lapping the lower fibers 64 it is possible that the distance from the top flat face 82 of the fiber cladding 79 and the fiber core 78 may be slightly different for different fibers 64. This would yield a different coupling ratio for the various fibers 64. By adjusting laterally the position of the upper fiber 95, the coupling ratio can be maintained constant at each switch position.

FIG. 21 shows a fixture for permitting accurate adjustment of the upper substrate 91 with respect to the lower substrate 66 to accomplish the previously described switching function. The quartz block 70 is fixed in a lower trough created by a pair of walls 102 and 104 of a frame 106. The wall 104 is slightly lower in height than the edge of the block 70 of the switch. A pair of ledges 108 above the walls 102 and 104 are wide enough to permit at least half the width of the substrate 91 to slide over them. An upper wall 110 supports two spring members 112 which maintain the upper substrate 91 biased away from the wall 110. A pair of micrometers 114 and 116 are mounted in the frame 106. The micrometer 114 bears upon the upper substrate 91 of the switch against a spring 112 and is adjusted to change the optical coupling from one fiber 64 to a different fiber 64. The micrometer 114 is centrally located on the side of the substrate 91 to permit this lateral adjustment without rotation of the substrate 91. The micrometer 116 is offset on the upper substrate 91 so that it can be used to adjust the angular orientation of the upper substrate 91 with respect to the lower substrate 70.

Using a construction quite similar to that described with respect to the fiber optic switch, a discretely variable optical delay line, shown in FIG. 22, can be made.

As with the switch, the upper substrate 91 of the optical delay line is made of quartz and has a v-groove 93 with a fiber 95 secured therein. This fiber is lapped to within a few microns of its core, just as the fiber in the switch.

The lower portion is formed with a quartz base 70 and a v-groove silicon substrate 66. The major difference is that there is only a single fiber 121 which is wrapped in helical fashion around the base 70 of the delay line. The first loop of fiber 121 is placed in the first v-groove 68. The next loop is placed in the next v-groove 68, and so on until all the v-grooves 78 are filled. When the two portions of the variable delay line are assembled in the manner described with respect to the switch, it can be seen that, by laterally adjusting substrate 91, the fiber 95 can be coupled with any one of the loops of the fiber 121. Thus, by moving substrate 91, different amounts of delay can be obtained. Thus, a discretely variable delay line has been described. The mechanism for changing the position of the upper substrate 91 of the delay line can take the form suggested previously in FIG. 21 for the switch.

FIG. 23 shows a graph of exemplary possible delays in the case of a 7-loop delay line of the type described above. The first graph shows the time at which an input light pulse is introduced into the fiber 121. The shortest delay is provided by aligning the upper fiber 95 with the tapped portion of the fiber 121 closest to the light source, and is illustrated in the next lower graph. The next three graphs depict the output of the delay line when the upper fiber 95 is aligned, respectively, with loops 3, 5 and 7 of the fiber 121. It can be seen that the difference between the delay at the first tap on the fiber 121 and the delay at the third tap on the fiber 121 is approximately 5 nanoseconds.

FIG. 24 shows schematically the use of the variable delay line of this invention in a tuneable filter. A modulated light source is shown as a laser diode input 123. Light from the diode 123 is coupled to the fiber 95 on the upper substrate 91 (FIG. 22) of the adjustable delay line. Each of the coupling points 125 along the length of the fiber 121 of the variable delay line provides a different delay as previously described. Fiber 95 can be adjusted to couple at any of the coupling points 125. In the filter shown in FIG. 24, the fibers 95 and 121 are polished so that the amount of coupling at each of these points 125 is 3 DB. Thus, half of the power input by the laser diode 123 will remain in fiber 95, while the other half will be coupled into fiber 121. Fibers 95 and 121 extend through another 3 DB directional coupler 127 which acts to recombine the two light signals for detection by a detector 129.

The filter shown in FIG. 24 has an impulse response consisting of two equal pulses separated in time by the relative delay nT-P where T is the delay of each loop between successive taps 125, n is the number of the loops between the coupling point 125 which is aligned with the fiber 95 and the coupler 127 and P is the difference in delay between the fibers connecting the coupler 127 to the coupling point on the fiber 95 and the last tap 125 of the fiber 121. The Fourier transform of this impulse response is a cosign function whose first zero is at F=1/(2nT-P). The filter in FIG. 24 will have this transfer characteristic provided the coherence length of the source 123 is short compared to the delay difference nT-P. Fixed filters of this type have been demonstrated with as many as 1,300 overtones or harmonics. FIGS. 25A-25C show the frequency response of the filter of FIG. 24. The upper trace in these figures is the frequency response of the laser diode 123 and the detector 129. The lower trace in FIG. 6A is the response for the minimum relative delay of 0.6 nanoseconds. An intermediate delay, 2.5-0.6=1.9 nanoseconds, is shown in FIG. 6B and the maximum delay of 15-0.6=14.4 nanoseconds is shown in FIG. 6C. The maximum delay in such discretely variable delay lines can be made very large since the attenuation and dispersion of optical fibers is low. The minimum delay between taps is limited to about 100 picoseconds by bending loss around the loop and by fabrication considerations. 

We claim:
 1. A fiber optic discretely variable delay line, comprising:a first single mode optical fiber having a core and a cladding, said first fiber having at least one tap; a second single mode optical fiber having a core and a cladding, said second fiber having plural taps, spaced at predetermined intervals along said second fiber, the cladding at each of the taps on said first and second fibers being sufficiently thin on at least one side thereof to tap light therefrom; and said first and second fibers selectively positionable relative to one another to permit the tap of said first fiber to be positioned in alignment with a selected one of said plural taps of said second fiber to form an optical coupler, whereby control of the transmission time of a light wave through said second fiber is provided by permitting the selection of a discretely variable delay for said light wave.
 2. A fiber optic discretely variable delay line, as defined by claim 1, further comprising:a first substrate for mounting said first fiber, said tap of said first fiber arranged at a surface of said first substrate; a second substrate for mounting said second fiber with said taps arranged along a surface of said second substrate; said substrates relatively positioned with said surfaces in a face-to-face relationship; and said substrates being slideable with respect to one another to permit said tap of said first fiber to be juxtaposed with a selected one of said plural taps of said second fiber.
 3. A fiber optic discretely variable delay line, as defined by claim 2, wherein the coupler formed by said juxtaposition of said tap of said first fiber with a selected one of said plural taps of said second fiber is an evanescent field coupler.
 4. A fiber optic discretely variable delay line, as defined by claim 2, wherein said second substrate includes plural grooves which mount said second fiber at said plural taps, respectively, and said first substrate includes a groove which mounts said first fiber at a said first fiber tap.
 5. A method of making a tapped delay line with a discretely variable time delay, said method comprising:providing first and second single mode optical fibers; removing a portion of the cladding from plural selected locations along said first fiber to provide respective plural evanescent field coupling regions; removing a portion of the cladding from a selected location along said second fiber to provide an evanescent field coupling region; and adjustably mounting said fibers for selective juxtaposition of said second fiber coupling region and any one of said plural first fiber coupling regions.
 6. A method of making a tapped delay line as defined in claim 5, wherein said mounting step comprises:wrapping said first fiber around a substrate to form plural loops; and positioning said plural coupling regions of said first fiber in respective grooves formed in said substrate.
 7. A fiber optic frequency filter, comprising:a first single mode optical fiber, a portion of which includes plural taps, spaced at predetermined intervals along said first fiber portion; a second single mode optical fiber; first means for optically coupling said second fiber to a selected one of said plural taps of said first fiber at a first predetermined location on said second fiber; second means for optically coupling said first and second fibers at a first predetermined location on said first fiber, removed from said first fiber portion, and at a second predetermined location on said second fiber; and means for adjusting said first means for optically coupling to couple said fibers at another of said plural taps to vary the frequency response of said filter.
 8. A fiber optic frequency filter, as defined by claim 7, wherein said first and second means for optically coupling each have a coupling ratio of substantially fifty percent.
 9. A fiber optic frequency filter, as defined by claim 7, additionally comprising:a light source for introducing light into one of said fibers to provide an optical output signal in the other of said fibers, wherein the coherence length of said light is less than the optical path difference of said fibers between said first and second coupling means.
 10. A method of frequency filtering, comprising:providing first and second single mode optical fibers each having a core and a cladding, a portion of said first fiber having plural taps, spaced at predetermined intervals along said first fiber and said first fiber having a first tap, the cladding at each of the taps on said first and second fiber being sufficiently thin on at least one side thereof to tap light therefrom; relatively positioning said optical fibers to provide optical coupling between said first and second fibers at one of said plural taps on said second fiber and at said first tap of said first fiber; an optical coupler coupling said first fiber to said second fiber at a predetermined location removed from said plural taps; introducing a modulated light wave into one of said single mode fibers to provide an optical output signal in the other of said single mode fibers, said output signal comprised of light modulated at first and second frequencies; and relatively positioning said first and second optical fibers to provide optical coupling at another of said plural taps selected to pass the portion of said optical output signal at said first frequency, while attenuating the portion of said optical output signal at said second frequency.
 11. A method of frequency filtering, as defined by claim 10, additionally comprising:selecting a coherence length for said light such that the optical path length difference for said first and second fibers between said another of said plural taps and said predetermined location removed from said taps is greater than said coherence length.
 12. A fiber optic switch, comprising:a set of plural strands of single mode optical fiber, each of said strands having an evanescent field coupling region; a strand of optical fiber, separate from said set of plural strands, having an evanescent field coupling region; and means for relatively positioning said optical fiber strand and said set of plural strands to selectively juxtapose said evanescent field coupling region of said optical fiber strand with a selected one of said evanescent field coupling regions of said set of plural strands, to selectively couple optical power between said fiber strand and one of said plural fiber strands.
 13. A fiber optic switch, as defined by claim 12, wherein said means for relatively positioning comprises:a substrate for mounting said set of plural fibers; a substrate for mounting said fiber strand; and means for positioning said substrates in face-to-face relationship, and for sliding said substrates relative to each other to provide for selective juxtaposition of said coupling regions. 